1. Field of the Invention
The present invention relates to elastic wave devices that propagate surface acoustic waves, boundary acoustic waves, or the like, and particularly relates to elastic wave devices that include a piston mode.
2. Description of the Related Art
Elastic wave devices capable of suppressing high-order transverse modes by exciting piston modes are known. For example, Japanese Unexamined Patent Application Publication No. 2013-544041 discloses a device in which a dielectric film is laminated on leading end portions of electrode fingers of an interdigital transducer (IDT) electrode. A portion where the dielectric film is laminated is referred to as an edge area, and an acoustic velocity in the edge area is slower than an acoustic velocity in a center area of a portion where the electrode fingers intersect with each other.
Further, Japanese Unexamined Patent Application Publication No. 2011-101350 cited below discloses a relationship of acoustic velocities in the center area and the edge area for realizing piston modes.
In Japanese Unexamined Patent Application Publication No. 2013-544041 and Japanese Unexamined Patent Application Publication No. 2011-101350, a piston mode is excited by adjusting a length as a dimension of the edge area along an elastic wave propagation direction and the acoustic velocity in the edge area. Accordingly, the length, width, and the like of the edge area for the excitation of the piston mode are defined for a single elastic wave propagation velocity.
In a longitudinally coupled resonator-type elastic wave filter including a plurality of IDTs, an elastic wave device provided with IDTs including narrow-pitch electrode finger portions, and the like, electrode finger pitches, metallization ratios, or the like are changed along the elastic wave propagation direction. As such, a method in which a length, width, and the like of the edge area are maintained to be constant, like in Japanese Unexamined Patent Application Publication No. 2013-544041 and Japanese Unexamined Patent Application Publication No. 2011-101350, is effective for a single elastic wave propagation acoustic velocity, but not effective in other portions where elastic waves propagate at a different velocity. Therefore, the above method cannot sufficiently suppress high-order transverse modes.